It has been recognized by Meisl et al. in their U.S. Pat. No. 4,551,754 entitled COMPATIBLE WIDE-SCREEN COLOR TELEVISION SYSTEM filed Feb. 18, 1982, which issued Nov. 5, 1985 that a wide screen television signal may be made compatible with conventional television receivers by compressing or "squeezing" the left and right edges of the wide screen image. When displayed on a conventional television receiver, the squeezed edges of the image are largely hidden from view due to receiver overscan. When displayed on a wide screen receiver, the compressed edges are restored to their original width by means of time expansion circuits. In one embodiment of the Meise et al. system, picture edge squeezing is provided by modifying the horizontal drive signal to a camera. In another embodiment, image restoration ("de-squeezing") is provided by means of a memory which stores the video signal in response to a constant frequency write clock and recovers the stored signal in response to a variable frequency read clock. Changing the read clock frequency alters the relative timing of picture elements within a horizontal line thereby facilitating expansion of the compressed edges of the displayed image.
To provide for display of both wide screen and standard aspect ratio (i.e., 4:3) images in a dual mode receiver, Meise et al. propose that a coded signal be added to the vertical blanking interval of the compatible (edge squeezed) wide screen signal for identifying the signal as being representative of a wide screen image. (As used herein, the term wide screen means any aspect ratio greater than 4:3 as used in conventional television displays.) The coded signal is detected in the dual mode receiver and used to control the display raster width and the edge expansion circuit. When the code is present, the edge expander circuits are enabled and the raster width is expanded to the full width of a wide screen kinescope. When standard television signals are received, the absence of the code is detected and used to reduce the raster width to provide a 4:3 aspect ratio and the edge expansion circuits are disabled (by-passed).
Similar edge expansion and raster width control arrangements are described in a further U.S. patent application of Dischert et al. entitled KINESCOPE BLANKING SCHEME FOR WIDE-ASPECT RATIO TELEVISION, Ser. No. 551,918 filed Nov. 15, 1983 which issued Dec. 3, 1985, as U.S. Pat. No. 4,556,906. The technique of varying the raster width in a receiver for displaying wide and standard aspect ratio images is further exemplified in a projection television system proposed by Shioda et al. in U.S. Pat. No. 4,385,324 entitled WIDE SCREEN PROJECTION APPARATUS which issued May 24, 1983. In the Shioda et al. system a coded signal is also employed for automatically controlling the raster size in a dual mode receiver.
Another example of a compatible wide-screen system is described by K. H. Powers in U.S. Pat. No. 4,605,952 entitled COMPATIBLE HDTV SYSTEM EMPLOYING NONLINEAR EDGE COMPRESSION/EXPANSION FOR ASPECT RATIO CONTROL which issued Aug. 12, 1986 on an application Ser. No. 504,374 filed June 14, 1983 as a continuation-in-part of application Ser. No. 485,446 filed Apr. 14, 1983 (now abandoned). In the Powers system the center portion of the image is slightly compressed (by about 2.5%) and the compression of the edges of the image ramps linearly to a factor of about 3:1 at the extreme edges. Edge compression is provided in the Powers system by the use of variable clock rate sampling of an analog video signal. The sampling rate is varied by applying the output of a very high frequency (4.374 Giga-Hertz) oscillator to a programmable divider having divisor coefficients stored in a programmable read only memory (ROM). The ROM is addressed by a counter that is clocked during each line interval thereby changing the divisor coefficients and thus changing the sampling frequency to control the edge compression of the sampled video signal.